from what i understand theres a pressure plate in the passenger seat that detects if someone is in the seat when the car starts, and if its below a certain weight (like a child seat with a child) it turns the passenger airbag off. if a full grown adult or nobody is there it stays on
I once put my laptop on the passenger seat, and thought I had a major problem because my pass. airbag light was on after I started my Max. Sensor works well! Took a simple moving of the laptop to the rear as I tried to figure out what was wrong. Realized that the light works by sensor.